The AMI image "ami-1c91086f" has the repository "https://sdkrepo.atlassian.com/debian" already configured in it which makes the availability of this repository mandatory if we have to run the "apt-get update" command. But, due to the recent announcement (https://community.developer.atlassian.com/t/atlassian-plugin-sdk/14813/3) made about the movement of the repo it seems the command on the image has started to fail:

13-Feb-2018 08:38:22 Err:11 https://sdkrepo.atlassian.com/debian stable/contrib amd64 Packages
13-Feb-2018 08:38:22 404 Not Found